All spell casters gain their abilities through a form of demon possession. So in order to gain magical abilites, PCs must track down and trap demons. Of course there are some pretty powerful demons, if the demon is stronger than the PC, the PC could find themselves subjugated to the Demon's will.
Trying to find a place for this...
"After the famine hit the land, the priest Galen began religious wanderings, drawing his congregation to follow him. Those who took up with him, began to walk, a great outpouring of energy to this religious pilgrimage, coming from an impoverished nation with no true direction other than to find relief. Galen professed to his followers that they would find great resources set by his God down on Earth for them, enough to nourish and slate their thirst. Those that fell as he took them across the barren plains of the western reaches, rose again to join the lines. When finally all had died, Galen led them further, praising Nuh-Erell for that first city they came upon and a feast of flesh that lay in wait"